[Feature request] Metric for last successful remote_addresses_url refresh #2056

Is your feature request related to a problem? Please describe.

We use remote_addresses_url with Google's crawler IP lists and cannot
monitor whether the lists are actually refreshed: /healthz does not reflect
the list state, and a successful refresh with an unchanged list is only
logged at debug level, so "refresh works, nothing new" and "refresh stopped
working" look the same.

Solution you would like.

A gauge per list:

anubis_remote_addresses_last_success_timestamp_seconds{url}: Unix time of
the last successful fetch, including fetches where the list was unchanged
(0 if no fetch has succeeded yet).

This allows a simple alert like
time() - anubis_remote_addresses_last_success_timestamp_seconds > 26*3600
and a "last update at" panel in dashboards.

Describe alternatives you have considered.

Alerting on WARN log lines (…giving up until next refresh window etc.).
It works for failures, but needs a log pipeline and cannot show the current
state.

Additional context

Related: #1965, #193.